Shrouded-coanda multiphase burner

ABSTRACT

A gas flare disposed at an end of a supply pipe member for directing a gas flow to a head member, the flare including a slot for ejecting the gas flow and a shroud. The shroud surrounds the slot, at least part of the base member and at least part of the head member of the gas flare. The shroud directs the ejected gas flow between the shroud and the head member.

FIELD OF THE DISCLOSURE

The disclosure relates to a gas flare, and in particular, but notexclusively to a Coanda-type gas burner flare used for combustion ofwaste gas during surface testing of hydrocarbon wells.

BACKGROUND

Hydrocarbons' importance for the world economy cannot be overstated. Thediscovery and efficient production of hydrocarbons is becomingincreasingly more difficult and poses many new technological challenges.Typically, a borehole is drilled down into the earth, on land or insubsea operations, to reach a reservoir containing hydrocarbons.Usually, fluid hydrocarbons take the form of oil, gas, or mixturesthereof found in reservoirs that can be produced by one or more wells.

An important stage in the evaluation of a reservoir is known as welltesting. Well testing includes flowing a well and measuring the responseof several key parameters such as pressure and flow rate over time. Manydifferent types of well tests are known to those skilled in the art,such as pressure drawdown, interference, reservoir limit tests, etc.Well testing enables the collection of data that help assess theeconomic viability of the well. However, the cost of testing operationsis significant and often times it may exceed the cost associated withdrilling the well. It is important, therefore, that testing operationsare performed as efficiently and economically as possible.

During a typical well testing operation, the well effluent is separatedinto its individual phases, i.e., oil, water and gas via the use of oneor more well test separators. One of the key functions of a well testingoperation is to combust the waste gas flow exiting the well testseparator.

Many different flaring systems have been used to burn the waste gasflow. One well known type of flare systems are the Coanda-type flaringsystems. Coanda-type flare systems are widely used in the petroleumindustry for flaring waste gases of oil refineries or productionplatforms. Typical Coanda-type flare systems are described in thefollowing patent documents: U.S. Pat. No. 3,709,654, U.S. Pat. No.3,915,622, U.S. 2006/0105276 A1, U.S. Pat. No. 3,833,37, EP 0054383 andRU 2315240. Coanda flares offer relatively clean combustion by premixingthe waste gas with ambient air prior to combustion. Coanda flares alsooffer generally good, stable flame combustion at high inlet pressure ofcombustible gas. In addition, shockwaves generated in the vicinity ofthe gas exit slot of a Coanda flare help to atomize the liquid dropletsthat are present in the gas flow into a fine mist, thus facilitatingcombustion and reducing the risk of fall out even in the case ofsignificant liquid carryover in flare line. So, generally Coanda-typeflaring systems offer stable and quite clean combustion of multiphasehydrocarbon effluents. Coanda-type flares are efficient andenvironmental friendly under proper operational conditions.

There are, however, some disadvantages that limit the use of Coandaflares in well test operations such as noise, ejected debris, andsensitivity to flow rate variations. The noise is believed to be mainlyassociated with shockwaves and is generally excessive and poses safetyand environmental issues.

The Coanda-type flare apparatus described in U.S. Pat. No. 4,486,167comprises a Coanda body, a slot for gas outlet form the supply pipe, afructoconical shield for noise reduction, and another shield for noisereflection. However, the addition of these shield decreases the airsupply to the burner and creates a narrow operational range on flowrates of gas.

Furthermore, there is also a high risk of debris being ejected radiallyfrom the Coanda slot in the form of high velocity projectiles. Forexample, the debris can be sand particles or other particulates presentin the wellbore effluent. Another source of debris is pipe scale or thebuilt up of salt deposits around the Coanda slot that can becomedislodged once they reach a critical mass. Debris ejected through theCoanda slot poses a risk of injury to the operators as well as damage tothe surrounding equipment. There may also be a fraction of the bigliquid droplets that do not follow the gas flow and spray sideways fromthe slot, causing some fallout, especially if the liquid present in gasstream is highly viscous. Moreover, during well test operations theremay be frequent and unexpected flow variations or variations in thecomposition of the hydrocarbon effluent (e.g. the liquid fraction in thegas stream) that can lead to a temporary cancelling of the Coanda effect(gas or liquid droplets not following the Coanda profile). Under theseconditions the gas/liquid mix sprays out radially causing unintendedside effects and dangers, as well as hydrocarbon spill.

It is therefore desirable to overcome the limitations of existingCoanda-type flaring systems, especially insofar as surface well testingoperations are concerned.

SUMMARY

According to a first aspect of the disclosure there is provided a gasflare that overcomes the above mentioned deficiencies of existingCoanda-type gas flares. The inventive gas flare comprises: a supply pipefor directing a gas flow to a head member; a slot for ejecting the gasflow; and a shroud surrounding the slot, at least part of the supplypipe and at least part of the head member. The shroud directs theejected gas flow between the shroud and the head member.

Advantageously, the shroud prevents the gas flow from being ejectedsubstantially radially from the gas flare. So while the gas flow isstill ejected radially from the slot to provide a Coanda effect andaspiration of an ambient air, such gas flow including any liquidprogresses only as far as the shroud and adheres to the head member. Bypreventing radial gas flow from the gas flare the safety of the deviceis improved, since any debris contained in the high-speed gas flow willbe contained within the shroud.

According to a further aspect of the disclosure there is provided ashroud for a gas flare, the shroud surrounding at least a portion of thegas flare and designed for directing a gas flow ejected from the gasflare to flow between the shroud and the gas flare and for enabling anambient air flow to be directed to mix with the gas flow ejected fromthe gas flare.

According to a further aspect of the disclosure there is provided ashroud with an axial airfoil-shaped cross-section with a smooth bottomend and a sharp distal end, so that the air aspired by gas jet comingfrom a slot is being accelerated from the smooth bottom end to the sharpdistal end of the shroud.

According to yet a further aspect of the disclosure there is provided amethod of combusting gas ejected from a gas flare; the methodcomprising: directing the gas flow using a gas flare having a contouredshape; directing an ambient air flow using a shroud surrounding at leastpart of the gas flare to flow between the shroud and gas flare; mixingthe gas flow with an ambient air flow; and combusting the mixture.

The disclosure could be used with other gas flare systems, however, itis particularly advantageous when used in combination with a Coanda-typeflare, especially one having a tulip-contoured shape.

According to yet a further aspect of the disclosure, the Coanda gasflare comprises a stalk-shaped base member connected to a tulip-shapedhead member. At least part of the stalk-shaped base member is locatedwithin at least part of a supply pipeline defining a first passagetherebetween for directing a gas flow towards the head member. TheCoanda gas flare further comprises a slot located between the supplypipeline and the head member, substantially adjacent to where the headmember is connected to the base member. The slot is surrounded by ashroud defining a second passage therebetween for directing the gas flowsubstantially around the tulip-shaped head member to be combusted.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described solely by way ofexample and with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art Coanda gas flare; and

FIG. 2 shows a cross-sectional view of a gas flare according to a oneembodiment of the disclosure;

DETAILED DESCRIPTION

So that the above features and advantages of the present disclosure canbe understood in detail, a more particular description of thedisclosure, briefly summarized above, may be had by reference to theembodiments thereof that are illustrated in the accompanied drawings. Itis to be noted, however, that the drawings illustrate only typicalembodiments of this disclosure and are therefore not to be consideredlimiting of its scope, for the disclosure may admit to other equallyeffective embodiments.

In a typical well testing operation a gas flare is used to burn and,thus, dispose waste gas effluent exiting a well test separator that isused to separate a multiphase hydrocarbon effluent into its oil, gas andwater components. Although the effluent to be combusted is typicallyreferred to as gas effluent, it should be appreciated that there isstill a fraction of liquid that often remains in the gas flow, which isto be combusted. This liquid fraction is especially high during theflowing of high gas rate fluid at the upper operation limit ofseparator, or flowing bypass separator on gas-condensate wells.

The Coanda-type flare is often useful to combust this liquid fraction,since the gas flow ejected from the Coanda-gas flare reaches supersonicspeeds. This creates shockwaves, which atomize the liquid fraction inthe gas flow into a fine mist that is more easily and completelycombusted by the burner flame.

FIG. 1 shows an example of a prior art Coanda flare having a supply pipe114 connected to a tulip-shaped head member 110 via a base member 112with pylons. The tulip shape produces the Coanda effect, which isadvantageous in that the waste gas flow 116 is directed around the bulge108 of the tulip which causes an accelerated gas flow in this area andthus produces low pressure. The gas flow is directed, by this lowpressure, adjacent to the surface of the tulip-shaped head member, andalso causes ambient air flow to be directed toward the low pressure.

Thus, ambient air flow mixes with the gas flow ejected from a slot 104of the gas flare, making the mixture 117 even more suitable forcombustion by the burner flame at the distal end of the head member 110.

The supply pipe 114 and base member 112 provides a first passagewaythere between having a particular cross-section through which the highpressured gas 116 flows. This high pressured gas flow would typicallycome from a well test separator gas outlet or directly from a testedwell. The slot 104 defines a second narrower passageway for choking theejected gas flow between the supply pipe 114 and the head member 110.Since the second passageway of the slot 104 is narrower, the gas flow isaccelerated to sonic speed within the slot, and furthermore (due toexpansion along with the tulip) the flow reaches supersonic velocities;this produces shockwaves that help to atomize the liquid dropletsentrained by the gas flow. Then, the tulip-shaped head of the Coandaflare causes ambient air flow to mix with the combustible gas flow. Thispremixing allows achieving a good fuel-oxidant ratio and makes the flamemore stable and clean.

FIG. 2 shows a cross-sectional view of a gas flare according to oneembodiment of the disclosure described herein. FIG. 2 shows a Coanda gasflare with base member 12 connected to the supply pipe 13. The basemember 12 may be at least partially located within a supply pipe 13. Thebase member 12 and supply pipe 13 are shaped so as to define apassageway there between through which the gas flow is directed towardsthe head member 10. As the gas flow reaches the head member, there is aslot 4 between the supply pipe 13 and the head member 10 and base member12. Specifically, the slot 4 defines a narrowed passageway foraccelerating and ejecting the gas flow outside of the gas flare.

The directed gas flow is ejected substantially radially from the slot 4.However, the gas flare has a shroud 14 which surrounds the slot and aportion of supply pipe and the bottom part 11 of the head member 10.More specifically, the shroud surrounds and yet is spaced a distancefrom the slot 4, to define a further passageway for the gas andaspirated air to flow around the contour of the tulip-shaped head member10 and to mix with each other,. This allows for shockwaves that aidliquid atomization, but furthermore acts as a shield in preventingparticulates in the gas flow from being ejected past the shroud—whichmay be of danger to working nearby personnel. Furthermore, it isbelieved that the containment of the shockwaves within the shroud 14 ispartially responsible for the observed noise reduction in the backwarddirection.

In one embodiment, the frusto-conical shroud 14 has an airfoil-shapedvertical cross-section with a sharp top edge 15 and a smooth bottom end16. This airfoil-shaped geometry of the shroud 14 adds additionalfunctionality to the shroud. The shroud 12 works as an efficient airejector toward the flow of gas emitted from the slot 4. The smooth andstreamlined geometry of the bottom end of the shroud 14 reduces the drageffects of the inlet air. The reduced area of the cross-section definedby the supply pipe 13 and shroud 14 induces acceleration of the inletair. The high velocity air flow detaches from the sharp top edge 15 ofthe shroud 14 and removes the portion of liquid droplets produced byatomization in the slot 4, that occasionally separate from the main gasstream.

Additionally, the top edge of shroud 14 may have a serrated rim 20. Theelements of serrated rim 20 are known in the designing of jet nozzles astabs (directed inward the streamlining flow) or chevrons (sharp cornerof the structure directed outward the flow). The function of thissmall-scale serration is for the redistribution and better atomizationof liquid jets occurring on the inside surface of shroud 14 and forimproving the mixing of different flows.

Although the geometry of tabs is depicted as small sharp triangles, itshould be appreciated that other shapes are possible: polygonalelements, combination of rounded elements with polygons, etc.

The shroud 14 is also able to absorb high-frequency noise, bandsresulting from the share layer and shockwaves, for example byconstructing the shroud with a sound absorption structure (porousmaterial or honeycomb) intermediary layer.

Additionally, the flare apparatus is equipped with a back-shield 17 fornoise attenuation in the backward direction of the flare apparatus. Inthe design shown in FIG. 2, the back-shield 17 has a slightly concavegeometry for improving the air inlet in the annulus defined by thebottom end 16 of the shroud and the back-shield 17. In otherembodiments, the back-shield may be flat plate attached to the supplypipe 13. Preferably, the back-shield 17 is performed fromsound-absorbing materials (similar to materials for shroud 14) with arigid back wall for additional noise redirection towards the flame area.

Thus, the shroud covers a portion of the Coanda flare and is able toperform multiple functions which improve the flare.

In addition to sound absorption, and hence noise suppression, the shroudmay act as a protective shield to capture any debris (sand, saltdeposits, etc.) that is ejected radially from the Coanda slot. It alsoprevents any gas or liquid from being sprayed radially in the case of anunexpected sudden flow rate spike or high liquid loading that causes theCoanda effect to break down. The directed and accelerated airflow willensure that any gas or liquid droplets, not following the Coandaprofile, will be redirected in an axial direction into the combustionzone.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred and alternativeembodiments of the present disclosure without departing from its truespirit. In addition, this description is intended for purposes ofillustration only and should not be construed in a limiting sense.

1. A gas flare comprising: a supply pipe for directing a gas flowtowards a head member; a slot located between the supply pipe and headmember for ejecting the gas flow; and a shroud spaced away from andsurrounding the slot to form a passage between the shroud and the headmember for directing the ejected gas flow therebetween, wherein theshroud has an airfoil-shaped longitudinal cross-section with a smoothbottom end and a sharp top edge.
 2. The gas flare of claim 1, whereinthe shroud prevents the gas flow from being ejected in a substantiallyradial direction from the gas flare.
 3. The gas flare of claim 1,wherein the shroud prevents debris in the gas flow from being ejectedfrom the gas flare.
 4. The gas flare of claim 1, wherein the shroudabsorbs noise.
 5. The gas flare of claim 1, wherein the shroud absorbs aselectable high-frequency band of the noise as the gas is ejected fromthe slot at a supersonic speed.
 6. The gas flare of claim 1, wherein theshroud comprises a porous inner material layer and a solid externalmetal lining.
 7. The gas flare of claim 1, further comprising: a basemember disposed near a bottom of the head member, wherein the basemember is at least partially located within the supply pipe defining apassage therebetween for the gas flow.
 8. The gas flare of claim 1,wherein the shroud includes a circumferential serrated rim at the distalend.
 9. A shroud for a gas flare, the shroud being at least partiallydisposed around a head member of the flare and comprising: a proximateend having a generally smooth and rounded shape; and a distal end havinga sharp edge, wherein the shroud and the head member define a passagefor directing a gas flow.
 10. A method of combusting a gas flow ejectedfrom a gas flare; the method comprising: directing the gas flow from asupply pipe to a head member; drawing in an ambient air flow into apassage formed by a shroud spaced away from and surrounding at leastpart of the gas flare; mixing the gas flow with the ambient air flow;and combusting the mixture.
 11. The method of claim 10 furthercomprising partially disposing the shroud between a shield and the headmember, wherein the shield provides a backward noise reduction.
 12. Themethod of claim 11 further comprising forming a smooth passage betweenthe shroud and the shield.
 13. A Coanda gas flare for combustingeffluent, the gas flare comprising: a stalk-shaped base member connectedto a tulip-shaped head member, wherein at least part of the base memberis located within at least part of a supply pipeline defining a firstpassage therebetween for directing a gas flow towards the head member;and a slot located between the supply pipeline and the head member,substantially adjacent to where the head member is connected to the basemember, wherein the slot is surrounded by a shroud defining a secondpassage therebetween for directing the gas flow substantially around thetulip-shaped head member to be combusted.